So-called meterless hydraulic control circuits control the motion of a hydraulic actuator by controlling a flow from one chamber of the actuator to the other utilizing one or more pumps, that is, flow rate from the pump(s) is used to control the flow to and/or from the chambers of the actuator, as opposed to utilizing proportional valves. Proportional or throttling valves are utilized in prior art metered systems to restrict or meter the fluid flow therethrough to control movement of the actuator. In contrast, in meterless systems, the pump(s) may be of a variable displacement type or of a fixed displacement type wherein the flow from the pump to the actuator chambers is varied in order to control the speed of the actuator movement. In prior art meterless arrangements, pump controlled circuits known as Displacement Controls (DC) utilize a variable displacement pump with a constant speed driver, while Electro-Hydrostatic Actuators (EHA) utilize a fixed displacement pump with a variable speed driver.
Inasmuch as flow to and from the chambers of the actuator is controlled by the pump, under certain conditions, unintended motion can happen when there is a change in load even though there is no operator command. For example, when the pump displacement is zero, the machine is turned off, or an implement lockout switch is set to disable hydraulic operation of the implement system, the load can sometimes slip to a different position. One such meterless hydraulic control circuit is shown, for example, in U.S. Publication 2009/0165450. In this arrangement, six solenoid valves are utilized to control the flow through various conduits, the pump and a tank. In order to hold the load, five solenoid valves must be maintained in the closed position.